1. Field of the Invention
The invention relates to a method of welding workpieces, between which there is a gap, at least in portions, in which
a) at least one signal corresponding to radiation issuing from the workpieces during the welding process is generated for monitoring and controlling the welding process;
b) the limit value MAX(B.sub.0), MIN(B.sub.0) of the radiation signal which must not be exceeded or fallen below with gap width zero for maintaining a weld of predetermined minimum quality is determined in a preliminary test;
c) the signal is compared with at least one predetermined limit value MAX(B), MIN(B) for checking the quality of the welding process and a fault signal is generated if this limit value is exceeded or fallen below.
2. Discussion of Relevant Art
A method of this type is known from U.S. Pat. No. 5,272,312. With this method, radiation which is generated by the workpieces owing to a machining laser beam is used to monitor and control the machining process. On the one hand, this radiation is usually ultraviolet light which issues from the steam capillaries generated in the workpieces by the machining laser beam, On the other hand, it is infrared radiation which is irradiated from the molten region following the steam capillaries when viewed in the direction of movement of the machining head but also the already consolidated region located after the weld point. Holes or open pores created by the spraying of material from the weld seam can be determined and documented by means of infrared radiation.
The signals typical of a fault can only be detected after the fault has been created. Therefore, these signals cannot be used for ruling out the cause of defective welds. However, these signals can be used to document the quality of the welding process. In fact, holes or open pores and therefore also the signals produced by them can be tolerated to a certain extent. For this reason, limit values for the signals which correspond to infrared or/and ultraviolet radiation and ensure a specific quality of welding can be determined, for example in preliminary tests, if the signals lie within these limit values. If the limit values are exceeded or fallen below during the welding process, a fault signal is generated. The real time evaluation of the fault signals during the welding process finally opens into a calculation of the probability that a significant welding fault has been created.
In particular during the welding of metal sheets or pipes in which there is a gap between the workpieces, it is difficult to monitor and control the quality of the welding process with the apparatus known from U.S. Pat. No. 5,272,312 The gap per se between the workpieces should have a minimum width so the welding process can be carried out as effectively as possible. In practice, however, the gap width varies and normally lies between 0 and 100 .mu.m. With a typical diameter of the weld point of about 250 .mu.m (laser beam of a Co.sub.2 laser focused by mirrors), a gap width of 100 .mu.m leads to a considerable loss of up to 50% of laser power which can be introduced into the workpieces.
Since the gap width modulates the power which can be introduced into the workpieces for melting the metal, the signals detected by the welding monitoring means and corresponding to the radiation issuing from the workpieces are also modulated by the gap width. However, the decline in the power which can be introduced into the workpieces and the associated weakening of the corresponding radiation signals does not necessarily mean the presence of a welding fault. To avoid the continuous display of welding faults during the welding of workpieces between which there is a gap, owing to the weld monitoring process used for quality control, the window limited by the limit values for the detected signals has to be set wider than necessary with a gap-free weld. However, the drawback is that it is sometimes impossible reliably to detect faults actually occurring in such a butt weld, for example owing to holes or open pores.